Te presence of an oxide layer a b c d
Fig. 5. Shown here are the secondary electron images of the AlN coating in (a and b) Steel A (8.8% Al and 1.1% Si) and (c and d) Steel B (8.8% Al and 1.6% Si) after nitriding for eight hours at 900C. The case depth and plate-like structure of the AlN is similar between the two different silicon containing steels, and the AlN is shown to precipitate and grow along specific crystal- lographic directions in the austenite (b and d). (d) Oxide layers that were determined to be most consistent with MnAl2O4 and Al2O3 developed on the surface of the 8.8% Al steels at an average depth of 10-15 µm after eight hours at 900C.
indicates that not all of the oxygen was eliminated from the furnace atmosphere. However, the oxide layer did not develop until after extended times during the nitriding process, and aluminum nitride formation was always favored because of the much higher solubility and diffusion of interstitial nitrogen in austenite over oxygen. Figure 7 shows the depth of the aluminum nitride layer as a function of time and temperature. Depending on the time and tem- perature, aluminum nitride coating thicknesses between 200 and 550 µm can be achieved. Steels A and B with 8.8% Al and 1.1% Si to 1.6% Si show similar coating thicknesses for all times and temperatures. Although silicon is known to
a b
decrease the solubility of nitrogen in steels, increasing the silicon level in the current study from 1.1% to 1.6% had little effect on the depth of the aluminum nitride layer or the appar- ent density of the plates precipitated. However, increasing the amount of aluminum from 6% to 8.8% produced a denser array of very fine plates but sharply decreased the depth of the aluminum nitride layer. It also produced a 50% reduction in the diffusivity of nitrogen and increased the activation energy from 64 to 78 kJ/mol in the temperature range of 1,652-2,012F (900-1,100C). Te results of the current study
c d
Fig. 6. Shown are the secondary electron micrographs of (a and b) Steel B and (c and d) Steel C after nitriding for eight hours at 1,100C. (a and b) In Steel B, the AlN consists as a high density of longer and typically thiner plates that grow in parallel packets in specific crystallographic direc- tions within the austenite. (c and d) In Steel C, the density of AlN in the reaction layer is much less. However the plate thickness is greater and case depth of AlN is almost 200 µm greater than in Steel B.
show that high manganese and alumi- num austenitic steels can be nitrided in a gaseous nitrogen atmosphere to produce a hard and wear-resistant layer of AlN at depths of up to 550 µm. Increasing the amount of silicon from 1.1% to 1.6%Si in a Fe-30Mn- 8.8Al-0.9C steel had no statistical effect on the diffusion of nitrogen in the temperature range of 900 to 1100C. However, increasing the aluminum content in high manganese steels increases the activity of nitrogen and reduces the solubility and diffusiv- ity of nitrogen. Tis article is based on the paper
“Nitriding of Lightweight High Man- ganese and Aluminum Steels” (15-036), originally presented at the 119th Metalcasting Congress.
June 2015 MODERN CASTING | 31
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